In order to understand the nature of the present invention, it is helpful to first review various aspects of a turbofan jet engine. Below is a brief description of the exhaust components and fire safety elements of a turbofan jet engine, followed by description of the airflow dynamics associated with these elements during use.
With regard to engine exhaust components, FIG. 1 is a forward-looking perspective rear view of a primary exhaust nozzle 20 and a primary nozzle plug 22 of a turbofan jet engine. The primary exhaust nozzle is generally positioned at the aft end of a gas generator (not shown). As illustrated in FIG. 3, the primary exhaust nozzle includes a nozzle fairing 26 concentrically attached to a nozzle body 24. The forward end of the nozzle inner sleeve is connected to the gas generator or to adjoining generator structure such as an engine turbine rear frame. An aft engine mount (not shown) is used to connect the engine to a support structure (see FIG. 1), e.g., a wing strut or pylon. The downstream primary nozzle components are not attached to the support structure so that during use the nozzle can accommodate aftward thermal expansion without contacting the support structure.
Referring to FIGS. 1 and 2, the support structure is a wing pylon covered by various fairings and heat shields. Referred to herein generically as "strut fairings" 30 are a first aft pylon fairing 32, an aft pylon fairing heat shield 34, a second aft pylon fairing 36, and various heat shield castings 38. Also shown is a thrust reverser inner wall 35, a thrust reverser outer wall 37, a thrust reverser aft cowl 39, and a lower bifurcation panel 40. An annular core compartment vent exit 42 is formed by the space between the nozzle fairing 26 and the thrust reverser aft cowl 39 to vent small amounts of bypass as well as gas generator cooling air. In addition, there may be other outer structure, such as nacelle or thrust reverser components, positioned near the primary nozzle. FIGS. 1-5 omit various of these other structures in order to show aspects and features of the present invention more clearly.
With regard to fire safety elements, a number of fire zones exist within the engine installation that are designed in a manner that prohibits an engine fire from spreading. Of particular interest to the present invention is an upper quadrant fire zone located just behind the aft engine mount, between the primary nozzle and the strut fairings. In FIG. 3, the aft mount fire zone is located generally along the nozzle at the location labeled 44. Commercial aircraft propulsion systems require each fire zone to be bounded by a fire seal that is capable of containing and isolating a fire, not only from other propulsion installation components (e.g., nacelles and engine fairings), but also from areas surrounding the propulsion installation (e.g., wings, fairings, and fuselage).
In the case of the aft mount fire zone 44, a fire seal (not shown) is located along an upper arcuate region of the nozzle and oriented to prohibit flame from spreading aft of the gas generator or outward to the surrounding structures. Known fire seals, e.g., turkey feather seals, provide a solid barrier between the upper area of the primary nozzle and the strut fairings so that flame cannot pass to downstream locations. This fire seal thus protects the nozzle, the support structure, the wing structures, the various fairings and shields, and the aircraft fuselage from fire emanating rearward from the gas generator.
As shown best in FIG. 1, a region exists between the primary nozzle fairing 26 and the strut fairings at a location directly behind the aft mount fire seal. This region is termed herein a "bounded low pressure region" 48. The forward end of the bounded low pressure region 48 is defined generally by the aft mount fire seal. The bounded low pressure region aft end is defined by the primary nozzle aft end.
With regard to airflow dynamics during engine operation, fan air 52 flows along either side of the strut fairings 30. The lower surface of the aft pylon fairing heat shield 34 and the nozzle are separated by approximately the height of the core compartment vent exit 42. A low velocity cooling flow issues from the core compartment vent exit. When the core vent flow encounters the aft mount fire seal, it separates and later converges downstream of the bounded low pressure region 48.
Because of the geometry of engine components and the dynamics of airflow, there exists regions of differing velocity and/or pressure in the areas of the primary nozzle. In particular, the fan airflow is a higher velocity on either side of the bounded low pressure region, while the velocity within the region itself is low since it is a base region that is somewhat protected from the fan air 52. This pressure/velocity difference results in side-to-side air fluctuations that superimpose on the rearward-directed fan flow. The fluctuations can eventually cause nozzle vibration which is amplified when the natural frequencies of the flow fluctuations and the nozzle vibration coincide.
In general, it is desirable to eliminate flow fluctuations and structural vibrations. Thus a need exists for an engine device that accomplishes these tasks. The present invention is directed to fulfilling this need.